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Date:   Fri, 24 Mar 2023 08:06:07 +0800
From:   Masami Hiramatsu (Google) <mhiramat@kernel.org>
To:     Beau Belgrave <beaub@linux.microsoft.com>
Cc:     rostedt@goodmis.org, mathieu.desnoyers@efficios.com,
        dcook@linux.microsoft.com, alanau@linux.microsoft.com,
        brauner@kernel.org, akpm@linux-foundation.org,
        ebiederm@xmission.com, keescook@chromium.org, tglx@linutronix.de,
        linux-trace-devel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCH v8 09/11] tracing/user_events: Update documentation for
 ABI
Message-Id: <20230324080607.37bf83f86ac9bfcaee675d95@kernel.org>
In-Reply-To: <20230221211143.574-10-beaub@linux.microsoft.com>
References: <20230221211143.574-1-beaub@linux.microsoft.com>
        <20230221211143.574-10-beaub@linux.microsoft.com>
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Hi Beau,

On Tue, 21 Feb 2023 13:11:41 -0800
Beau Belgrave <beaub@linux.microsoft.com> wrote:

> The ABI for user_events has changed from mmap() based to remote writes.
> Update the documentation to reflect these changes, add new section for
> unregistering events since lifetime is now tied to tasks instead of
> files.

Thanks for updating the document. This is good. And I would like to
ask you to update one more thing in the document.

In "the Writing Data" Section,

-----
For example, if I have a struct like this::

  struct payload {
        int src;
        int dst;
        int flags;
  };
-----

This payload has to be packed too.

This may be a split patch as a fix because it is not related to this
series.

Thank you,

> 
> Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
> ---
>  Documentation/trace/user_events.rst | 177 ++++++++++++++++------------
>  1 file changed, 102 insertions(+), 75 deletions(-)
> 
> diff --git a/Documentation/trace/user_events.rst b/Documentation/trace/user_events.rst
> index 9f181f342a70..0180714f10e3 100644
> --- a/Documentation/trace/user_events.rst
> +++ b/Documentation/trace/user_events.rst
> @@ -11,20 +11,19 @@ that can be viewed via existing tools, such as ftrace and perf.
>  To enable this feature, build your kernel with CONFIG_USER_EVENTS=y.
>  
>  Programs can view status of the events via
> -/sys/kernel/debug/tracing/user_events_status and can both register and write
> -data out via /sys/kernel/debug/tracing/user_events_data.
> +/sys/kernel/tracing/user_events_status and can both register and write
> +data out via /sys/kernel/tracing/user_events_data.
>  
> -Programs can also use /sys/kernel/debug/tracing/dynamic_events to register and
> +Programs can also use /sys/kernel/tracing/dynamic_events to register and
>  delete user based events via the u: prefix. The format of the command to
>  dynamic_events is the same as the ioctl with the u: prefix applied.
>  
>  Typically programs will register a set of events that they wish to expose to
>  tools that can read trace_events (such as ftrace and perf). The registration
> -process gives back two ints to the program for each event. The first int is
> -the status bit. This describes which bit in little-endian format in the
> -/sys/kernel/debug/tracing/user_events_status file represents this event. The
> -second int is the write index which describes the data when a write() or
> -writev() is called on the /sys/kernel/debug/tracing/user_events_data file.
> +process tells the kernel which address and bit to reflect if any tool has
> +enabled the event and data should be written. The registration will give back
> +a write index which describes the data when a write() or writev() is called
> +on the /sys/kernel/tracing/user_events_data file.
>  
>  The structures referenced in this document are contained within the
>  /include/uapi/linux/user_events.h file in the source tree.
> @@ -35,29 +34,70 @@ filesystem and may be mounted at different paths than above.*
>  Registering
>  -----------
>  Registering within a user process is done via ioctl() out to the
> -/sys/kernel/debug/tracing/user_events_data file. The command to issue is
> +/sys/kernel/tracing/user_events_data file. The command to issue is
>  DIAG_IOCSREG.
>  
>  This command takes a packed struct user_reg as an argument::
>  
>    struct user_reg {
> -        u32 size;
> -        u64 name_args;
> -        u32 status_bit;
> -        u32 write_index;
> -  };
> +        /* Input: Size of the user_reg structure being used */
> +        __u32 size;
> +
> +        /* Input: Bit in enable address to use */
> +        __u8 enable_bit;
> +
> +        /* Input: Enable size in bytes at address */
> +        __u8 enable_size;
> +
> +        /* Input: Flags for future use, set to 0 */
> +        __u16 flags;
> +
> +        /* Input: Address to update when enabled */
> +        __u64 enable_addr;
> +
> +        /* Input: Pointer to string with event name, description and flags */
> +        __u64 name_args;
> +
> +        /* Output: Index of the event to use when writing data */
> +        __u32 write_index;
> +  } __attribute__((__packed__));
> +
> +The struct user_reg requires all the above inputs to be set appropriately.
>  
> -The struct user_reg requires two inputs, the first is the size of the structure
> -to ensure forward and backward compatibility. The second is the command string
> -to issue for registering. Upon success two outputs are set, the status bit
> -and the write index.
> ++ size: This must be set to sizeof(struct user_reg).
> +
> ++ enable_bit: The bit to reflect the event status at the address specified by
> +  enable_addr.
> +
> ++ enable_size: The size of the value specified by enable_addr.
> +  This must be 4 (32-bit) or 8 (64-bit). 64-bit values are only allowed to be
> +  used on 64-bit kernels, however, 32-bit can be used on all kernels.
> +
> ++ flags: The flags to use, if any. For the initial version this must be 0.
> +  Callers should first attempt to use flags and retry without flags to ensure
> +  support for lower versions of the kernel. If a flag is not supported -EINVAL
> +  is returned.
> +
> ++ enable_addr: The address of the value to use to reflect event status. This
> +  must be naturally aligned and write accessible within the user program.
> +
> ++ name_args: The name and arguments to describe the event, see command format
> +  for details.
> +
> +Upon successful registration the following is set.
> +
> ++ write_index: The index to use for this file descriptor that represents this
> +  event when writing out data. The index is unique to this instance of the file
> +  descriptor that was used for the registration. See writing data for details.
>  
>  User based events show up under tracefs like any other event under the
>  subsystem named "user_events". This means tools that wish to attach to the
> -events need to use /sys/kernel/debug/tracing/events/user_events/[name]/enable
> +events need to use /sys/kernel/tracing/events/user_events/[name]/enable
>  or perf record -e user_events:[name] when attaching/recording.
>  
> -**NOTE:** *The write_index returned is only valid for the FD that was used*
> +**NOTE:** The event subsystem name by default is "user_events". Callers should
> +not assume it will always be "user_events". Operators reserve the right in the
> +future to change the subsystem name per-process to accomodate event isolation.
>  
>  Command Format
>  ^^^^^^^^^^^^^^
> @@ -94,9 +134,9 @@ Would be represented by the following field::
>    struct mytype myname 20
>  
>  Deleting
> ------------
> +--------
>  Deleting an event from within a user process is done via ioctl() out to the
> -/sys/kernel/debug/tracing/user_events_data file. The command to issue is
> +/sys/kernel/tracing/user_events_data file. The command to issue is
>  DIAG_IOCSDEL.
>  
>  This command only requires a single string specifying the event to delete by
> @@ -104,92 +144,79 @@ its name. Delete will only succeed if there are no references left to the
>  event (in both user and kernel space). User programs should use a separate file
>  to request deletes than the one used for registration due to this.
>  
> -Status
> -------
> -When tools attach/record user based events the status of the event is updated
> -in realtime. This allows user programs to only incur the cost of the write() or
> -writev() calls when something is actively attached to the event.
> -
> -User programs call mmap() on /sys/kernel/debug/tracing/user_events_status to
> -check the status for each event that is registered. The bit to check in the
> -file is given back after the register ioctl() via user_reg.status_bit. The bit
> -is always in little-endian format. Programs can check if the bit is set either
> -using a byte-wise index with a mask or a long-wise index with a little-endian
> -mask.
> -
> -Currently the size of user_events_status is a single page, however, custom
> -kernel configurations can change this size to allow more user based events. In
> -all cases the size of the file is a multiple of a page size.
> +Unregistering
> +-------------
> +If after registering an event it is no longer wanted to be updated then it can
> +be disabled via ioctl() out to the /sys/kernel/tracing/user_events_data file.
> +The command to issue is DIAG_IOCSUNREG. This is different than deleting, where
> +deleting actually removes the event from the system. Unregistering simply tells
> +the kernel your process is no longer interested in updates to the event.
>  
> -For example, if the register ioctl() gives back a status_bit of 3 you would
> -check byte 0 (3 / 8) of the returned mmap data and then AND the result with 8
> -(1 << (3 % 8)) to see if anything is attached to that event.
> +This command takes a packed struct user_unreg as an argument::
>  
> -A byte-wise index check is performed as follows::
> +  struct user_unreg {
> +        /* Input: Size of the user_unreg structure being used */
> +        __u32 size;
>  
> -  int index, mask;
> -  char *status_page;
> +        /* Input: Bit to unregister */
> +        __u8 disable_bit;
>  
> -  index = status_bit / 8;
> -  mask = 1 << (status_bit % 8);
> +        /* Input: Reserved, set to 0 */
> +        __u8 __reserved;
>  
> -  ...
> +        /* Input: Reserved, set to 0 */
> +        __u16 __reserved2;
>  
> -  if (status_page[index] & mask) {
> -        /* Enabled */
> -  }
> +        /* Input: Address to unregister */
> +        __u64 disable_addr;
> +  } __attribute__((__packed__));
>  
> -A long-wise index check is performed as follows::
> +The struct user_unreg requires all the above inputs to be set appropriately.
>  
> -  #include <asm/bitsperlong.h>
> -  #include <endian.h>
> ++ size: This must be set to sizeof(struct user_unreg).
>  
> -  #if __BITS_PER_LONG == 64
> -  #define endian_swap(x) htole64(x)
> -  #else
> -  #define endian_swap(x) htole32(x)
> -  #endif
> ++ disable_bit: This must be set to the bit to disable (same bit that was
> +  previously registered via enable_bit).
>  
> -  long index, mask, *status_page;
> ++ disable_addr: This must be set to the address to disable (same address that was
> +  previously registered via enable_addr).
>  
> -  index = status_bit / __BITS_PER_LONG;
> -  mask = 1L << (status_bit % __BITS_PER_LONG);
> -  mask = endian_swap(mask);
> +**NOTE:** Events are automatically unregistered when execve() is invoked. During
> +fork() the registered events will be retained and must be unregistered manually
> +in each process if wanted.
>  
> -  ...
> +Status
> +------
> +When tools attach/record user based events the status of the event is updated
> +in realtime. This allows user programs to only incur the cost of the write() or
> +writev() calls when something is actively attached to the event.
>  
> -  if (status_page[index] & mask) {
> -        /* Enabled */
> -  }
> +The kernel will update the specified bit that was registered for the event as
> +tools attach/detach from the event. User programs simply check if the bit is set
> +to see if something is attached or not.
>  
>  Administrators can easily check the status of all registered events by reading
>  the user_events_status file directly via a terminal. The output is as follows::
>  
> -  Byte:Name [# Comments]
> +  Name [# Comments]
>    ...
>  
>    Active: ActiveCount
>    Busy: BusyCount
> -  Max: MaxCount
>  
>  For example, on a system that has a single event the output looks like this::
>  
> -  1:test
> +  test
>  
>    Active: 1
>    Busy: 0
> -  Max: 32768
>  
>  If a user enables the user event via ftrace, the output would change to this::
>  
> -  1:test # Used by ftrace
> +  test # Used by ftrace
>  
>    Active: 1
>    Busy: 1
> -  Max: 32768
> -
> -**NOTE:** *A status bit of 0 will never be returned. This allows user programs
> -to have a bit that can be used on error cases.*
>  
>  Writing Data
>  ------------
> -- 
> 2.25.1
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>